Lysosomal storage disorders are a group of autosomal recessive genetic diseases characterized by the accumulation of cellular glycosphingolipids, glycogen, or mucopolysaccharides within intracellular compartments called lysosomes. Individuals with these diseases carry mutant genes coding for enzymes which are defective in catalyzing the hydrolysis of one or more of these substances, which then build up in the lysosomes. For example, Pompe disease, also known as acid maltase deficiency or glycogen storage disease type II, is one of several lysosomal storage disorders. Other examples of lysosomal disorders include Gaucher disease, GM1-gangliosidosis, fucosidosis, mucopolysaccharidoses, Hurler-Scheie disease, Niemann-Pick A and B diseases, and Fabry disease. Pompe disease is also classified as a neuromuscular disease or a metabolic myopathy.
Pompe disease is estimated to occur in about 1 in 40,000 births, and is caused by a mutation in the GAA gene, which codes for the enzyme lysosomal α-glucosidase (EC:3.2.1.20), also commonly known as acid α-glucosidase. Acid α-glucosidase is involved in the metabolism of glycogen, a branched polysaccharide which is the major storage form of glucose in animals, by catalyzing its hydrolysis into glucose within the lysosomes. Because individuals with Pompe disease produce mutant, defective acid α-glucosidase which is inactive or has reduced activity, glycogen breakdown occurs slowly or not at all, and glycogen accumulates in the lysosomes of various tissues, particularly in striated muscles, leading to a broad spectrum of clinical manifestations, including progressive muscle weakness and respiratory insufficiency. Tissues such as the heart and skeletal muscles are particularly affected.
Pompe disease can vary widely in the degree of enzyme deficiency, severity and age of onset, and over 500 different mutations in the GAA gene have been identified, many of which cause disease symptoms of varying severity. The disease has been classified into broad types: early onset or infantile and late onset. Earlier onset of disease and lower enzymatic activity are generally associated with a more severe clinical course. Infantile Pompe disease is the most severe, resulting from complete or near complete acid α-glucosidase deficiency, and presents with symptoms that include severe lack of muscle tone, weakness, enlarged liver and heart, and cardiomyopathy. The tongue may become enlarged and protrude, and swallowing may become difficult. Most affected children die from respiratory or cardiac complications before the age of two. Late onset Pompe disease can present at any age older than 12 months and is characterized by a lack of cardiac involvement and better short-term prognosis. Symptoms are related to progressive skeletal muscle dysfunction, and involve generalized muscle weakness and wasting of respiratory muscles in the trunk, proximal lower limbs, and diaphragm. Some adult patients are devoid of major symptoms or motor limitations. Prognosis generally depends on the extent of respiratory muscle involvement. Most subjects with Pompe disease eventually progress to physical debilitation requiring the use of a wheelchair and assisted ventilation, with premature death often occurring due to respiratory failure.
Recent treatment options for Pompe disease include enzyme replacement therapy (ERT) with recombinant human acid α-glucosidase (rhGAA). Conventional rhGAA products are known under the names alglucosidase alfa, Myozyme® or Lumizyme®, from Genzyme, Inc. ERT is a chronic treatment required throughout the lifetime of the patient, and involves administering the replacement enzyme by intravenous infusion. The replacement enzyme is then transported in the circulation and enters lysosomes within cells, where it acts to break down the accumulated glycogen, compensating for the deficient activity of the endogenous defective mutant enzyme, and thus relieving the disease symptoms. In subjects with infantile onset Pompe disease, treatment with alglucosidase alfa has been shown to significantly improve survival compared to historical controls, and in late onset Pompe disease, alglucosidase alfa has been shown to have a statistically significant, if modest, effect on the 6-Minute Walk Test (6MWT) and forced vital capacity (FVC) compared to placebo.
However, the majority of subjects either remain stable or continue to deteriorate while undergoing treatment with alglucosidase alfa. The reason for the apparent sub-optimal effect of ERT with alglucosidase alfa is unclear, but could be partly due to the progressive nature of underlying muscle pathology, or the poor tissue targeting of the current ERT. For example, the infused enzyme is not stable at neutral pH, including at the pH of plasma (about pH 7.4), and can be irreversibly inactivated within the circulation. Furthermore, infused alglucosidase alfa shows insufficient uptake in key disease-relevant muscles, possibly due to inadequate glycosylation with mannose-6-phosphate (M6P) residues. Such residues bind cation-independent mannose-6-phosphate receptors (CIMPR) at the cell surface, allowing the enzyme to enter the cell and the lysosomes within. Therefore, high doses of the enzyme may be required for effective treatment so that an adequate amount of active enzyme can reach the lysosomes, making the therapy costly and time-consuming.
There are seven potential N-linked glycosylation sites on rhGAA. Since each glycosylation site is heterogeneous in the type of N-linked oligosaccharides (N-glycans) present, rhGAA consist of a complex mixture of proteins with N-glycans having varying binding affinities for M6P receptor and other carbohydrate receptors. rhGAA that contains a high mannose N-glycans having one M6P group (mono-M6P) binds to CIMPR with low (˜6,000 nM) affinity while rhGAA that contains two M6P groups on same N-glycan (bis-M6P) bind with high (˜2 nM) affinity. Representative structures for non-phosphorylated, mono-M6P, and bis-M6P glycans are shown by FIG. 1A. The mannose-6-P group is shown by FIG. 1B. Once inside the lysosome, rhGAA can enzymatically degrade accumulated glycogen. However, conventional rhGAAs have low total levels of M6P- and bis-M6P bearing glycans and, thus, target muscle cells poorly resulting in inferior delivery of rhGAA to the lysosomes. Productive drug targeting of rhGAA is shown in FIG. 2A. The majority of rhGAA molecules in these conventional products do not have phosphorylated N-glycans, thereby lacking affinity for the CIMPR. Non-phosphorylated high mannose glycans can also be cleared by the mannose receptor which results in non-productive clearance of the ERT (FIG. 2B).
The other type of N-glycans, complex carbohydrates, which contain galactose and sialic acids, are also present on rhGAA. Since complex N-glycans are not phosphorylated they have no affinity for CIMPR. However, complex-type N-glycans with exposed galactose residues have moderate to high affinity for the asialoglycoprotein receptor on liver hepatocytes which leads to rapid non-productive clearance of rhGAA (FIG. 2B).
Current manufacturing processes used to make conventional rhGAA, such as Myozyme®, Lumizyme® or alglucosidase alfa, have not significantly increased the content of M6P or bis-M6P because cellular carbohydrate processing is naturally complex and extremely difficult to manipulate. Accordingly, there remains a need for further improvements to enzyme replacement therapy for treatment of Pompe disease, such as new manufacturing, capturing and purification processes for rhGAA.
Similarly, other recombinant proteins that are targeted to the lysosome, such as other lysosomal enzymes, also bind CIMPR. However, current manufacturing processes used to make other conventional recombinant proteins that are targeted to lysosomes do not provide recombinant proteins with a high content of M6P or bis-M6P. Accordingly, there remains a need for further improvements in the manufacturing, capturing and purification processes for these other recombinant proteins as well.